Current pump laser packages typically include an optical sub-assembly that comprises a laser chip and an optical fiber that acts as a wave guide, both of which are mounted on a platform in proper optical alignment with each other. Often, the method employed to effect this alignment will depend on the degree of precision required (less precision is required for surface emitting lasers than for edge emitting lasers). For sub-assemblies comprising surface emitting lasers, where less precision is required, the alignment may be effected passively (e.g., using solder bumps and wettable pads, as taught, for example, in U.S. Pat. No. 5,499,312).
Bumps of solder (and other materials) have been used also to provide electrical conductivity in optical devices. Such use is disclosed, for example, in U.S. Pat. No. 5,689,279.
For applications where greater precision of alignment is required, as is the case for edge emitting laser chips, the alignment is effected actively, by activating the laser and moving it relative to the optical wave guide until maximum transmission is achieved. Once the alignment of laser to fiber is achieved, both the laser and the optical fiber are fixed to a platform, which is, in turn, mounted on a base, typically using solder to form a rigid joint. The fiber also is fixed to the package by using either welding or a solder joint to form a hermetic seal. The package base, in turn, is generally bolted to a rigid external platform. Such active alignment and bonding techniques are described in U.S. Pat. No. 5,700,987, the contents of which are hereby incorporated herein by reference.
The joint between the optical sub-assembly platform and the base serves as the main means for dissipating heat generated by the laser when it is being operated. Because of other structural requirements, the portion of the platform supporting the laser and the portion to which the fiber sheath is attached usually are formed of different materials. For example, the portion of the platform on which the laser chip is mounted can be beryllium oxide (BeO) whereas the portion of the platform to which the optical fiber sheath is attached may be Kovar. Also, the package base to which the optical sub-assembly platform is joined typically is made of yet another material, for example, copper/tungsten (CuW). Each of these materials has a different coefficient of expansion from that of the other materials. Upon heating or cooling of the package, the differential expansion or contraction causes stresses that tend to distort the sub-assembly platform. Any such distortion would tend to cause misalignment of the laser and fiber, thereby reducing the efficiency of the device. Similar stresses are typically created when the package base is bolted to an external platform. These stresses, too, tend to cause similar misalignment.